The present invention relates to the signal processing of radar video signals and specifically to a means of deriving the correct threshold level from the average magnitude of echo return signals which may be used to eliminate clutter returns at precise pre-established false alarm rates.
When an object is present in the space which is scanned by a radar system, an echo returns as a return pulse in response to each of a certain number of radiated microwave pulses either to the antenna or to another similarly controlled antenna for use in receiving such a return signal from each unit azimuth region. In order to facilitate detection of a target, namely, an object to be detected, a receiver output signal produced by a receiver of the radar system is usually used to produce a visual display in which the target is included.
In practice, it is not seldom that at least one spurious object is present in the scanned space regardless of presence and absence of the target. Besides a target return or echo, namely, the return pulse from the target, such spurious objects produce spurious returns, which result in clutter in the visual display. Although the spurious objects are often referred to as a clutter, the work "clutter" will be used in the instant specification primarily to mean the spurious returns irrespective of utilization or not of a visual display. As the case may be, signals produced in the receiver from the target return and the clutter will be called a target return and clutter, respectively.
Examples of the clutter for a radar are land or ground clutter resulting from buildings and undulating terrains, sea clutter arising from sea surface, weather clutter originating with rainfall and rain clouds, and angel echoes attributed to other foreign matters, such as large flocks of migratory birds and/or atmospheric discontinuity or hererogeneity. The difference between the target return and the clutter depends on the field of use of the radar.
Since we are not interested in terrain echo returns or flocks of birds etc., it is desirable to eliminate clutter returns without adversely affecting desired target returns. This task has been alleviated to some extent by the prior art techniques given by the following patents:
U.S. Pat. No. 3,829,858 issued to Bergkvist on Aug. 13, '74, PA0 U.S. Pat. No. 4,318,101 issued to Musha et al on Mar. 2, '82, PA0 U.S. Pat. No. 4,249,177 issued to Chen on Feb. 3, '81, PA0 U.S. Pat. No. 4,386,353 issued to Bleijerveld et al on May 31, '83, and PA0 U.S. Pat. No. 4,360,811 issued to Cantwell et al on Nov. 23, '82.
The most pertinent of the prior art techniques are the disclosures of the Musha and Cantwell patents which describe radar video signal processing systems which provide a constant false alarm rate in the presence of Rayleigh or non-Rayleigh (Weibull) distributions of clutter returns.
The coverage of the radar system, in all these prior art techniques, is divided into zones made up of azimuth sectors and range rings. The zones are further divided up into cells formed by the radar scans in range levels. Musha and Cantwell use a sophisticated cell averaging technique to derive a threshold level from the average echo strength in each zone. Finally, the threshold level is applied to a gate circuit through which received radar video signals are passed in the individual cells of each relevant zone only if the radar video signals exceed the threshold level.
The patent of Bleijerveld et al describes a radar video signal processing system which calculates a new average threshold value with each change in clutter distribution.
The Chen and Bergkvist patents show other state of the art radar video signal processing systems with means to vary the threshold levels in accordance with variations in clutter distribution.
In view of the foregoing discussion it is apparent that there currently exists the need for a system of processing radar video signals which derives the correct clutter threshold level which may be used to eliminate clutter returns at pre-set false alarm rates. The correct threshold level must vary with the changes of clutter distribution observed with each set of received radar return signals and must be capable of being varied as the false alarms are changed. The present invention is directed towards satisfying that need.